A number of different tools and types of electrical equipment require three-phase electrical power having medium voltage, ranging from about 1,500 to 4,160 volts, to operate. Such tools and equipment include, for example, fans, motors, and submersible pumps. When such tools experience failure, they can be difficult and expensive to repair and/or replace. Furthermore, while they are being repaired and/or replaced, other systems dependent on them may be rendered inoperable, thereby resulting in down time, and lost productivity and profits.
Failure of the foregoing tools and equipment commonly results from electrical failure, such as from transient voltage spikes and surges in electrical power used to operate such tools and equipment. A spike occurs when an amount of voltage which is higher than normal in a line occurs for a very short period of time (e.g., less than 50 microseconds). A surge occurs when an amount of voltage which is higher than normal in a line persists for an extended period of time (e.g., more than 50 microseconds). The term "surge" will be used herein to refer to both spikes and surges. Surges may be caused by many different factors, such as static electricity, (e.g., lightning, dust storms, wind, and the like), tree limbs falling on power lines, a car hitting an electric pole, inductive load switching (e.g., turning on and off electrical equipment), and the like. Because there are a number of possible sources of surges which can cause electrical equipment and tools to fail, it is important to be able to suppress such voltage surges.
Conventionally, transient voltage surges to medium voltage electrical equipment are controlled by primary arrestors set at primary voltages ranging, for example, from about 10,000 volts to about 15,000 volts. Such voltage settings are too high for sensitive medium voltage equipment and, as a result, such equipment is not reliably protected and is vulnerable to electrical damage and failure.
Accordingly, a continuing search has been directed to the development of transient voltage surge suppressors which respond quickly enough and absorb sufficient energy to prevent medium voltage electrical equipment from being damaged.